Boston, MA. “Bionic energy from ourselves” was the title of a presentation by Huai-An Chin, Ph.D. candidate of the Electrical Engineering Department at Princeton University, at the BIOMEDevice conference this week. Bionics, he said, is the application of characteristics in nature to the study and design of systems and devices.
Mechanical systems, he said, can augment human behavior, restoring lost capabilities. A hearing aid is an example, as are smart prosthetics, operating at the human-machine interface. Yet another example is the pacemaker, which despite its lifesaving capabilities requires a battery change every five to fifteen years. Can we find energy sources that would prolong the battery life?
Indeed we can, he suggested, by harvesting bioelectric power from the motions of everyday life. The process of exhaling, he said, can generate 0.4 W. In fact, one large-scale human energy-harvesting system is in place—Stockholm Central Station harvests body heat from the 250,000 people passing through the station daily to heat a nearby office building, shaving 25% off the electricity bill, Chin said.
But rather than focusing on body heat, Chin and his team at the McAlpine Research Group at Princeton are concentrating on piezoelectric and triboelectric effects.
The piezoelectric effect results in the generation of a voltage when a crystalline structure, with bio-interfaced devices able to generate 0.82 µW/cm2 at 100 Hz. Chin cited experiments with flexible piezoelectric material embedded in the lung of a cow, suggesting possibilities for intra-body energy harvesting. Even at the cellular level, he said, piezoelectric nanoribbons have shown the ability to generate potentially useful levels of energy. He cited some specific figures: 1.5 nN of force at the cellular membrane results 1-nm deflection, which can result in meaningful levels of energy production.
He envisioned a wearable device that in the near future might enable you to—while doing pushups—not just build muscle but generate power.
He next turned his attention to the triboelectric effect, which generates voltage when two dissimilar materials come into contact. The triboelectric effect can in fact be a nuisance in test-and-measurement applications, when the flexing of insulated conductors generates voltages not related to the device under test. However, the triboelectric effect can generate useful levels of energy.
For all such energy-harvesting applications, he concluded, challenges remain: efficiency, now at slightly better than 1%, needs to be improved to 20% or more; efficient circuits need to be designed to rectify the AC voltages produced by current energy-harvesting techniques; large-area fabrication methods need to be perfected to lower costs; and wireless connectivity is needed to improve convenience.